457 replies to this topic

[Cooperman]

You really don't need to add extra machininc cost for machining pistons. The 21253 do not need that for a road engine.
If you seriously don't want the nominal 10.2:1, which most would be happy with, you can take a bit more out of the combustion chambers when doing the head modifications.
The comp ratio and cam just needs to be matched with the correct distributor curve and an Aldon Yellow should work well.

[JonnyAlpha]

You really don't need to add extra machininc cost for machining pistons. The 21253 do not need that for a road engine.
If you seriously don't want the nominal 10.2:1, which most would be happy with, you can take a bit more out of the combustion chambers when doing the head modifications.
The comp ratio and cam just needs to be matched with the correct distributor curve and an Aldon Yellow should work well.

 

Yes - that's the route I am now going down. 21253's with the standard 8cc bowl. The head won't be getting done until probably the end of the year, by which time I'll know what CC I need in each chamber.

I am potentially looking at re-using the original Metro Electronic Ignition, refurbished and re-curved to suit? I will of course weigh the cost of that up against a new distributor. I see the Aldon Yellow comes in a Non Vacuum and Vacuum format, which would I need and why? 

[JonnyAlpha]

Ok so the block went off to the machinist. He gave me a call earlier in the week to say that he had re-ground the crank 0.010" (10 thou) on the mains and big end journals (we had discussed this previously as the journals were outside of the recommended wear. He also said that he had bored out the first cylinder (the worst of the four) and even after 0.020" removed there is still a rust mark on part of the bore. We agreed that I would have a look before continuing (as I was away on holiday).

 

So I will take a look but it seems as if this will now need +40 or even +60.

 

I contacted Mini Spares who have said that as along as the pistons have not been used then they will happily replace them for the larger size once known

[JonnyAlpha]

Just been re-checking my CR calculations for my own sanity.

Can someone check my calculations below and let me know if there are any errors. I am also not sure about the Swept Volume, according to Vizards Bible the bore size of a 1275 + 40 is 1312 however other research states that this is actually a 1310, so what do you use for CR calculation 1312 or 1310?

Also remembered finding some figures listed by AC Dodd which states:

 

"+040" - 1311cc bore - 2.821"/71.65mm"

 

Based on an early MG Metro (1989 Mk2) 1275 +40 Thou Rebore (1312) (2.821 Inches / 71.65mm) and standard 1275 A+ Crank Stroke of 81.3mm

 

A - Piston to Top Deck Height (0.010" / 0.0254mm) =  1.02cc

B - Piston Dish (P21253) = 8cc

C - Cylinder Head Chamber (To be confirmed) = 21.4cc

D - BK450 Head Gasket Compressed Volume = 3.8cc

E - P21253 Ring Landing = 0.75cc 

 

Swept Volume (SV) = 1312 / 4 = 328cc 

 

A+B+C+D+E = Unswept Volume (UV) = 34.97

 

CR = (SV+UV) / UV 

 

CR = (328cc + 34.97) / 34.97 

 

CR = 10.38

 

So to calculate the chamber capacity required for the cylinder head:

 

For a CR of 10:1

CR - 1 (10 - 1 = 9)

 

Required Swept Volume = SV / CR -1 = 328 / 9 = 36.44

 

36.44 - (A+B+D+E)

36.44 - 13.57 = 22.87cc

 

For a CR of 9.75:1

CR - 1 (9.75 -1) = 8.75

 

Required Swept Volume = SV / CR - 1 = 328 / 8.75 = 37.49

 

37.49 - (A+B+D+E)

37.49 - 13.57 = 23.92

 

So based on the above calculations for a CR or 10:1 I need to ensure that the Cyl Head chamber volume is 22.87 is increased by 1.47cc and for a CR of 9.75:1 it needs to be 23.92 i.e. increased from standard by 2.52cc. 

 

So the question now is when modifying a 12G940 cylinder head how much with the chamber capacity be increased by. If not by 2.52cc then I need to get the pistons machined.

[GraemeC]

The chamber capacity will be increased to whatever you need (within reason). Mine is 25cc so the figures you’re quoting should be easy enough to do.

[ACDodd]

It's normal for chamber capacities to be in the 22 to 28cc range for road engines depending on the spec.

Just calculate what you need an a head specialist will be able to size appropriately.

Having checked your math, the bk450 is 3.3cc. I make it 10.51:1 with 21.4cc head chamber. You need 24.4cc head chamber with a 0.010" deck height.
If you optimise the deck height by setting flush +recommend ed good build practice) then increase the chamber volume to 25.5cc to maintain 9.75:1.

Ac

Edited by ACDodd, 18 August 2019 - 09:52 AM.

[JonnyAlpha]

Just going back to a question on page two. When I split the block from the gearbox there was a hole filled with silastic. There was I believe a matching hole in the gearbox? Any ideas what this is and what I should do about it?

 

Here's the pic again for reference:

 

[Rorf]

That's the second bleed hole from the pressure release valve - it shouldn't be blocked.

Edited by Rorf, 27 August 2019 - 01:45 PM.

[JonnyAlpha]

Could be just another bolt hole for gearbox to engine block.

 

We had a look at it today and it appears to sit in line with a channel in the side of the block that has been blanked off?

[Rorf]

Someone posted this great pic a few days ago. Actually doesn't look as if it has any negative effect if filled there with RTV as the gearbox flange is there any way.

Attached Files

•  DyyjvFQ.jpg   59.45K   5 downloads

Edited by Rorf, 27 August 2019 - 01:51 PM.

[JonnyAlpha]

Well just been up to the machine shop and as mentioned earlier, because there was still a rust mark after +20, the machinist has bored and honed all four cylinders to +40.

I am about to send the +20 pistons back for exchange.

 

What I need to clarify with the machine shop is the required clearance. Before starting I stated (as advised elsewhere in these forums and in an email from Mini Spares) the total clearance should be 0.025" - 0.03" (thousandth of an inch). I mentioned this again today and the machinist said that they had been bored to the correct distance based on fitting Nural Pistons ?? He did not say what it was. For my peace of mind I need to check this before I start the build.

 

We also talked about next steps - trial build - I asked him if he had some machined down gudgeon pins to avoid having to press fit the pistons. He suggested that we should press fit them onto the supplied pins and take the rings off to do the trial build, he also said that you only needed to do one piston anyway. 

He is an old skool experienced machinist and has done a lot of mini engine builds but I am responsible for the build so I need to check.

 

Here's the block post rebore:

 

[Rorf]

One tends to forget the detail - I completed my engine build about 6 weeks ago but here is a pic of the bottom of my block; one can see a small brass plug, which is probably under your blob of silicon

 

I had edited my previous comment whilst you were busy replying.

Attached Files

•  Engine block bottom.JPG   76.14K   10 downloads

[JonnyAlpha]

Someone posted this great pic a few days ago. Actually doesn't look as if it has any negative effect if filled there with RTV as the gearbox flange is there any way.

 

What a stroke of luck. 

Looks like an oilway that goes to the thread on the Oil Pressure Switch?

In the edited pic below I have marked what I think are the correct locations with the two red crosses.

The top one had silastic in it and the bottom one on the side of the block looks to have been brazed closed. You can just make it out in the pic I posted above of the bored block, a brazed circle below the Oil Pressure Switch hole.

 

Should I be concerned / do anything about this?

 

[Rorf]

Nothing to be concerned about, both holes are closed off with brass plugs.

[JonnyAlpha]

Nothing to be concerned about, both holes are closed off with brass plugs.

 

Brilliant - many thanks